The folks at Our Little Thing blog have created a wonderful example of a pit greenhouse built with earthbags, PVC pipe, some wood, and plastic cover. This is the second one they have built, having learned many things from the first trial. In the harsh Patagonian climate, they plan to grow many things that are otherwise impossible to grow there, such as avocados and grapes. This seems like a very practical and inexpensive way to provide a large greenhouse space. You can see many more pictures and read more about how they built it on their blog. If you find this interesting, you might also see this recent blog post.
I had considered they might espalier the tree also, but according to my research avocados grow too fast/vigorous to use that pruning technique. Works well on apples, pears and plums though. The tangerine they hope to grow might be suitable for the greenhouse as there are probably some dwarf varieties. Both species in the most common commercial varieties get way to large.. As a kid I grew up in one of the largest citrus/avocado growing areas in California. Used to help pick and smudge..
Makes me wonder if avocado can be espaliered. Espalier is an old, old way of using thermal mass to push the hardiness zone for fruit trees/shrubs. Prune a marginally-hardy tree up against a sun-warmed, thick brick or stone wall. Or earthbag, of course! This would work nicely in a pit greenhouse, backing the espaliered trees up against a taller/deeper wall at the back of the greenhouse.
http://en.wikipedia.org/wiki/Espalier
That should work. Good idea.
I like this design, but just wonder how they are going to keep an avocado tree from growing thru the roof.. Even the dwarf varieties get 8-10ft tall. : )
Bonsai avocado?
This greenhouse seems way more practical than the walipini design that I reported on a few days ago. I like it a lot. This concept could really catch on.
Paul: What are the daytime and nighttime temperatures inside? Did you use heavy duty greenhouse film? Did you cover the earthbags?